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Republicans are not happy about the Democrats' attempt to pass the treaty by the end of the year

The US Senate has voted to open a debate on the new Strategic Arms Reduction Treaty (Start) between the US and Russia.

The agreement restricts each nation to a maximum of 1550 deployed warheads, a cut of about 30% from a limit set eight years ago.

The American and Russian presidents signed the treaty in April.

But its ratification in the Senate has been delayed by disagreements between Democrats and Republicans.

On Wednesday, the Senate voted 66-32 to begin debate on the treaty, with 55 Democrats, nine Republicans and two independents supporting the move.

It is believed that Democrat Evan Bayh - who missed this session - will provide the 67th vote needed for a two-thirds majority to ratify the accord.

Nuclear uncertainty

The treaty would also limit to 700 the number of deployed ballistic missiles or nuclear bombers .

Perhaps more significantly, it would establish a new inspection regime.

Since the previous Start treaty expired in December 2009, Russia and the US have not been able to conduct inspections of each other's nuclear stockpiles - leading to uncertainty about what the other side is doing.

This Senate has done its homework on New Start, and it is this Senate that must vote on itSenator John Kerry, Foreign Relations Committee

Getting the accord ratified has been a key part of President Barack Obama's attempts to "reset" relations with Russia.

Republicans have raised a number of concerns, including whether the treaty would allow the modernisation of America's nuclear arsenal.

Senator Kit Bond, the most senior Republican on the Senate Intelligence Committee, condemned the treaty as "giving Russia essentially a vote on our missile defence decisions".

Delaying tactics

Some Republicans have threatened to use delaying tactics to prevent the treaty being ratified until the new year, when the new Congress convenes.

While the Democrats will still control the Senate, they would have a harder time mustering a two-thirds majority.

There have also been complaints from the Republican side that there is not enough time to to fully debate the issue before the end of the year.

The Democrats say the pact has been available for review since April.

John Kerry, chairman of the Senate Foreign Relations Committee, said 60 current senators had participated in about 20 briefings or hearings on the treaty and had put some 900 questions to the administration - all of which had been answered.

"This Senate has done its homework on New Start, and it is this Senate that must vote on it," he said.

Several days of debate are planned, with a final vote early next week.